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Gas hot water tank as backup heat source in power outage?

Just wondering whether there are any safe, simple and cost effective ways that a homeowner in a cold climate could use the heat produced by an atmospheric vented hot water heater as an emergency heat source during a power outage.  With recent power outages in US and Canada, that c/b a good thing.

I was looking at the local gas/plumbing codes and it looks like a homeowner can use a hot water tank for both potable water and space heating, as long as (1) the HWT is rated for both potable and space heating purposes, (2) the combined length of the supply and return pipe to the heat emitter is 50 feet or less, (3) potable water circulates through the whole heating circuit at least once every 24 hours, and (4) all components in the heating circuit have never been exposed to toxic chemicals.  While a heat exchanger would qualify as the heat emitter for code purposes, I'm looking for a simpler/lower-cost alternative.

At first glance, as long as one buys an atmospheric vented hot water tank rated for both potable and space heating purposes (e.g. John Woods 75 gallon light duty commercial), it looks like one could run 3/4 inch supply and return piping to a big old radiator, perhaps in central location on  main floor to create gravity fed circulation.  And then use the valves in the big old radiator to keep potable water circulating through that circuit back to the HWT.  While not ideal, some heat is way better than no heat when it's minus 25 degrees Celsius outside (which is about -13 degrees Fahrenheit).

Helpful suggestions/tips, particularly from those who've done something similar, much appreciated.  Thanks.

BTW, primary heat source in residence is a natural gas furnace that requires electricity from the local power company to run.